Ninety-eight pregnancies in 57 patients with mechanical (ball or tilting disc) prosthetic heart valves are described. Sixty-one pregnancies developed in 36 patients with a mitral prosthesis, 12 pregnancies in 9 patients with an aortic prosthesis, 17 pregnancies in 10 patients with both mitral and aortic prostheses and 8 pregnancies in 2 patients with mitral and tricuspid prostheses. All patients were in I or in II NYHA class at conception and no significative change was noted during pregnancy. All the patients were treated with oral anticoagulant therapy at conception. Different kinds of antithrombotic and antiembolic prophylaxis were employed. There were 13 voluntary interruptions of pregnancy and 37 spontaneous abortions. In the 47 newborn 2 malformation complications (1 warfarin syndrome, 1 cleft palate) and 4 haemorrhagic complications (without sequelae) were noted. Two fatal thromboses of a mitral prosthesis occurred; systemic embolic complications were noted in 7 cases. These observations suggested different incidences of complications regarding the antithrombotic and anti-embolic prophylaxis. The authors point out the high risk of thrombotic and embolic complications and the low rate of successful outcome of pregnancy in these patients.